[stoppingdown]

The way I make sure that I'm not corrupting stuff is to periodically scan all the photo files and compute their fingerprint (MD5 or SHA1, whatever). I've written a small application for it (but, for instance, on the Mac it can be done by means of a simple script). What I get is a flat file with results such as:

 

MD5(20170415-0015.ARW)=7b57a7871f72367d66779ef9a1d372f5

MD5(20170415-0023.ARW)=5fd1ddab9a16432010cb541dced9073d

MD5(20170415-0030.ARW)=d89004332d7d88c840baf422bea54005

MD5(20170415-0033.ARW)=e2af7ae9a4911725a40a77f90d4fc4c6

MD5(20170415-0034.ARW)=9060475a512e9076e2db7b844a99a0a7

MD5(20170415-0042.ARW)=b80a83022e1da906637a090e95e05738

 

Thanks to the very nature of RAW files, those fingerprints should _never_ change. I just compare the file with the one from the previous scan to verify that everything is ok.

 

In the past, especially with magnetic media, a medium annoyance was the time needed to complete the scan: very long, and I typically launched it before going to sleep. With the speed of the embedded SDD in the latest MBP, 500 GB of photos are scanned in 20/30 minutes, and I can use the laptop to do other things in the meantime. The same approach can be used to verify whether multiple backups on external HDDs or optical media are fine (even though, for optical media, I'm applying the plan of recreating them every 8 years - BTW, checking the integrity before recreating an optical backup is important, otherwise I'd get rid of a good backup and replace it with a backup of a corrupted file).

 

I decided this approach when I discovered that an old MBP had silently corrupted a few photos - fortunately nothing important. Disk checks didn't detect the problem: you had to open the files to see it.

 

Some advanced filesystems, such as ZFS, do basically the same kind of check in background, but they are not available on Mac OS X or Windows. This feature might be included in a future new filesystem for Mac OS, but I'm not sure about when.

 

Unfortunately this approach doesn't work with JPG and DNG: Lightroom, with these kind of files, stores editing instructions directly inside the original file and there is no option to force it to use a sidecar .XMP...